Fabrication of metal oxide ultra-thin films having a nano-scale film thickness and having a good insulating property and a good dielectric property is actively studied in various fields from their broad applicability and from the industrial demand for them. For example, in the current semiconductor industry, it is an important technical theme to further increase the degree of integration of one layer in large-scale integrated circuits. For this, it is necessary and indispensable to make an insulating capacitor, one constitutive element of a circuit, of nanometer-level thin films. In recent ultra-micropatterned CMOS devices, some device parameters are now being to their practicable limit. Accordingly, development of ultra-thin film materials of high-dielectric-constant oxides capable of being substitutable for silicon oxide gate-insulating films in MOS transistors, low-dielectric-constant insulating film materials capable of being introduced into the space of multi-level Cu wiring structures, and processes for producing them could be an important breakthrough to the high-speed, power-saving and high-integration devices that are coming in future.
In the field of optoelectronics expected for its industrialization in the coming days, a coating technique with a dielectric thin film material of metal oxide having good reflection efficiency is desired and studied, and also in this, it is an important technical theme to develop a nano-level, accurate and uniform thin film material of metal oxide having a relatively high dielectric constant and a process for producing it.
Further, as a microwave modulation element in high-speed communication devices such as mobile phones and radio-LAN-related devices, a metal oxide capacitor having a varying dielectric constant is used, and the capacitance of the capacitor itself has a significant influence on its properties. Specifically, also in this field, there may be a greatly increasing demand for metal oxide thin films that are thinner and have a higher dielectric constant.
Accordingly, since the application fields of dielectric thins films of metal oxides extend extremely broadly, the level of the dielectric constant thereof required varies depending on their application fields, and the value may range from 1.5 to 100 or more. Therefore, the necessary condition common to the process of producing such dielectric thin films of metal oxides is that a nano-level thin and dense film of a metal oxide having a good insulating property is produced with good thickness accuracy and, at the same time, the oxide thin film is produced under nano-level control of its composition and structure for the purpose of designing the dielectric property of the film in any desired range in accordance with the object thereof. In addition, it is desirable that the process is inexpensive and energy-saving, not producing a large environmental load.
Heretofore, a method mentioned below is known for producing an ultra-thin film of metal oxide. For example, it is known that, in a method of growing a thin film from a vapor phase, such as CVD or PVD, a thin film having a thickness in an order of a few μm can be relatively readily produced (G. D. Wilk, et al., J. Appl. Phys., 89, p. 5243 (2001)). However, when a thin film having a nanometer-range thickness is desired to be produced according to the method, then the economical load to the apparatus itself for the production may be large and there may occur various problems in point of the material cost, the energy consumption and the exhaust gas treatment. Accordingly, it could not always be said that the producibility in the production method as a whole may be good. In addition, the method is not always suitable for producing a thin and uniform film on a substrate having a complicated form or on a substrate having a large area. Further, in the film production method, in general, the substrate temperature must be kept at a high temperature, and therefore the method is further problematic in that the metal oxide/substrate interface reaction may form some interfacial phase and it may worsen the dielectric property of the ultra-thin film of oxide produced.
Recently, a novel thin film production method referred to as ALD has been proposed, in which vapor phase growth and surface reaction are combined. For example, when two types of reactive gases such as titanium tetrachloride and hydrogen peroxide are alternatively introduced into a reactor, then an oxide thin film having an atomic-level thickness can be formed one after another. In such an ultra-thin film production process combined with surface reaction, a uniform thin film can be formed on a substrate of any and every form, in principle (M. Ritala, et al., Science, 288, p. 319 (2000)). However, so far as a vapor-phase method is employed, it is difficult to dramatically improve the energy efficiency and the material balance in the production process. In addition, since the raw materials usable in the process are limited, it is extremely difficult to design the composition and the structure of the metal oxide thin films to be produced, in any desired manner in accordance with a broad application range of the produced films, and to control the electric properties of the produced films.
A sol-gel process is often used in a method of producing metal oxide thin films. The method comprises optionally adding water and an organic solvent to a metal alkoxide, and applying the resulting solution onto a substrate in mode of dipping or spin coating to thereby form a thin film having a thickness of a few μm. In this case, however, since the thickness of the gel coating film is determined depending on the hydrodynamic physical data of the sol such as the viscosity and the density thereof, it is extremely difficult to form a uniform thin film having a thickness of 0.1 μm or less. In addition, it is also difficult to uniformly control the film composition and, as a result, the mechanical strength of the film obtained is extremely low and this causes the reduction in the breakdown voltage of the film. This problem is a fatal defect in practical use of dielectric materials, and in fact, there is no example of using the ultra-thin film of metal oxide produced according to the method for an insulator capacitor material. The same problem is pointed out also in a coating and drying method of using a metal oxide colloid.
Further, a method of producing a metal oxide thin film from deposition of a supersaturated solution of an inorganic ion, and a method of producing an oxide thin film on the surface of an electrode in an electrochemical process are studied (T. P. Niesen, J. Electroceram., 6, p. 169 (2001)). In these methods, a particulate or rod-like structure is formed on a solid surface, and a metal oxide thin film having a uniform nano-level thickness could not be obtained.
On the other hand, a surface sol-gel reaction method is known as a type of a wet process, which comprises contacting an excess amount of a metal compound having a group capable of reacting with a hydroxyl group for condensation and capable of producing a hydroxyl group through hydrolysis, with a solid having a surface hydroxyl group in a mode of chemical adsorption, then removing the excess metal compound from the solid surface, and thereafter hydrolyzing the metal compound existing on the solid surface to thereby form a deposition of an ultra-thin layer of a metal oxide gel thereon (JP-A 9-241008). In this method, since the thickness of the oxide thin film to be formed is defined by the adsorption saturation of the metal compound on the solid surface, or that is, by the amount of the hydroxyl group existing in the solid surface, an ultra-thin film of metal oxide gel having a predetermined thickness can be formed and, when the operation is repeated, then the metal oxide gel film can be formed with good thickness accuracy one after another. However, since the oxide gel thin film obtained according to the above-mentioned method has a low density and since it has a remaining organic functional group, and the film is problematic in that it readily undergoes dielectric breakdown. Accordingly, the ultra-thin film of metal oxide gel obtained according to the method could not be directly used for a dielectric insulating film material.
When the problems with the above-mentioned prior-art production methods and dielectric insulating materials are taken into consideration, then the matter of importance in a novel method for producing a dielectric insulating thin film is that a dielectric insulating thin film is produced with nano-level accuracy and with good producibility from a general and inexpensive metal oxide precursor by the use of a simple production apparatus and that the method itself may accept the control of the composition, the structure and the thickness of the thin film produced therein in order that the film produced can satisfy the requirement for the properties of various dielectric insulating thin films in any desired manner. Further, it is also important that the novel dielectric insulating material comprises the above-mentioned metal oxide thin film and has a desired dielectric constant.
Given that situation, the present invention has been made in consideration of the above-mentioned various problems, and an object of the invention is to provide a method for producing a dielectric insulating thin film, which is simple in its operation and which may produce a general-purpose dielectric insulating thin film having a varying dielectric constant and accepting an accurate film thickness control and a control of the composition, the structure and the thickness of the film produced. Another object of the invention is to provide a dielectric insulating thin film having a desired relative dielectric constant and a desired leakage current characteristic, and to provide a dielectric insulating material containing the dielectric insulating thin film.